1. Field of the Invention
The present invention relates to an inverter apparatus that rectifies a three-phase AC voltage of an AC input power supply using a rectifying circuit, converts the rectified voltage into a three-phase AC voltage having predetermined frequency and amplitude, on the basis of an output voltage command signal, using an inverter circuit that is subjected to pulse width modulation control, and supplies the three-phase AC voltage to a load.
2. Description of the Related Art
FIG. 9 is a circuit diagram illustrating an inverter apparatus according to the related art that converts a three-phase AC voltage from an AC input power supply 1, which is a commercial power supply, into a three-phase AC voltage having a predetermined frequency and supplies the converted three-phase AC voltage to an AC motor 2, which is a load.
In an inverter apparatus 10 shown in FIG. 9, reference numeral 11 indicates a rectifying circuit composed of a three-phase bridge rectifier circuit including diodes for rectifying the three-phase AC voltage of the AC input power supply 1. Reference numeral 12 indicates an electrolytic capacitor for smoothing the voltage rectified by the rectifying circuit 11. Reference numeral 13 indicates an inverter circuit that is formed by connecting anti-parallel circuits, each composed of a diode and a transistor, in a three-phase bridge manner and performs pulse width modulation (PWM) control, which will be described later, on a voltage formed between both ends of the electrolytic capacitor 12 to generate a three-phase AC voltage having predetermined frequency and amplitude. Reference numeral 14 indicates a frequency setting unit that sets the frequency of the three-phase AC voltage output from the inverter circuit 13. Reference numeral 15 indicates a voltage command generating unit that generates a three-phase sine wave output voltage command signal vi having a frequency corresponding to a frequency command value fi set by the frequency setting unit 14 and an amplitude corresponding to the generated frequency. Reference numeral 16 indicates a PWM controller that compares the output voltage command signal vi from the voltage command generating unit 15 with, for example, a chopping wave carrier signal and generates an inverter control signal whose pulse width is modulated to control the on or off states of the transistors of the inverter circuit 13.
As shown in FIG. 10, the voltage command generating unit 15 includes a three-phase sine wave oscillator 51, multipliers 52 to 54, and a voltage pattern generator 63. The three phase sine wave oscillator 51 of the voltage command generating unit 15 generates a three-phase sine wave voltage having a constant amplitude and a frequency corresponding to the frequency command value fi set by the frequency setting unit 14. In addition, for example, when the AC motor 2 is controlled such that a primary voltage (V) thereof is proportional to a primary frequency (f) (V/f constant control), when a torque boost compensation function at a low speed is additionally provided to the AC motor 2 in the V/f constant control, or when the AC motor 2 for driving a load having a square diminishing torque load characteristic is controlled, the voltage pattern generator 55 outputs a voltage amplitude command value va corresponding to the frequency command value fi input from the frequency setting unit 14, according to a voltage pattern indicating the relationship between an output voltage Vo and a predetermined output frequency fo of the inverter circuit.
The voltage command generating unit 15 multiplies the voltage amplitude command value va output from the voltage pattern generator 63 by a voltage vf corresponding a three-phase sine wave frequency output from the three-phase sine wave oscillator 51 by using the multipliers 52 to 54 to generate a three-phase sine wave voltage command signal vi having a frequency and an amplitude respectively corresponding to the frequency command value fi set by the frequency setting unit 14 and the voltage amplitude command value va generated by the voltage pattern generator 63. Then, the voltage command generating unit 15 outputs the three-phase sine wave voltage command signal vi to the PWM controller 16.
In the inverter apparatus 10 shown in FIG. 9 according to the related art, the smoothing capacitor 12 needs to have sufficiently large capacitance to smooth the voltage rectified by the rectifying circuit 11 into a DC voltage. Therefore, an electrolytic capacitor is used as the smoothing capacitor 12.
However, as known in the art, since the electrolytic capacitor is a part that is consumed with time, it has a shorter life span than other components. When the electrolytic capacitor is used in the inverter apparatus 10 for a long time, it is necessary to replace the smoothing capacitor 12 composed of the electrolytic capacitor with a new one in order to maintain the performance of the inverter apparatus, which results in an increase in maintenance costs.
In order to solve this problem, an inverter apparatus disclosed in JP-A-3-277180 includes a capacitor for absorbing a noise current caused by the switching operation of an inverter circuit, in addition to the electrolytic capacitor, in order to prevent the noise current from flowing through the electrolytic capacitor. According to the inverter apparatus disclosed in JP-A-3-277180, since the current flowing through the electrolytic capacitor is reduced, a load applied to the electrolytic capacitor is reduced, as compared to the conventional inverter apparatus shown in FIG. 9, which makes it possible to lengthen a life span. However, in this case, the overall size of the inverter apparatus increases due to the added capacitor for absorbing the noise current.
Further, JP-A-5-103494 discloses an inverter apparatus including only a smoothing capacitor having small capacitance in a rectifying circuit, without using the electrolytic capacitor.
In the inverter apparatus disclosed in JP-A-5-103494, since the smoothing capacitor has small capacitance, a high ripple voltage is included in a DC voltage rectified by a DC intermediate circuit. When the DC voltage varies due to the ripple voltage, a beat phenomenon occurs in an output current from the inverter apparatus due to the variation in the DC voltage.
In order to prevent the beat phenomenon of the output current caused by the variation in the DC voltage output from the DC intermediate circuit, the inverter apparatus disclosed in JP-A-5-103494 detects a voltage between both ends of the smoothing capacitor, and corrects a voltage amplitude command signal or a frequency command signal input to the inverter circuit, on the basis of the detected voltage.
However, the inverter apparatus disclosed in JP-A-5-103494 has a problem in that the maximum output voltage of the inverter apparatus is restricted by the ripple voltage included in the DC voltage from the DC intermediate circuit and an insufficient voltage is output from the inverter apparatus in a section in which the output voltage is high.